The Coolest Stars Come Out of the Dark

The Coolest Stars Come Out of the Dark

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-210&cid=release_2010-210

Astronomers have uncovered what appear to be 14 of the coldest stars known in our
universe. These failed stars, called brown dwarfs, are so cold and faint that they'd be
impossible to see with current visible-light telescopes. Spitzer's infrared vision was able to
pick out their feeble glow, much as a firefighter uses infrared goggles to find hot spots
buried underneath a dark forest floor.

The brown dwarfs join only a handful of similar objects previously discovered. The new
objects are between the temperatures of about 450 Kelvin to 600 Kelvin (350 to 620
degrees Fahrenheit). As far as stars go, this is bitter cold -- as cold, in some cases, as
planets around other stars.

These cool orbs have remained elusive for years, but will soon start coming out of the
dark in droves. NASA's Wide-field Infrared Survey Explorer (WISE) mission, which is
up scanning the entire sky now in infrared wavelengths, is expected to find hundreds of
objects of a similarly chilly disposition, if not even colder. WISE is searching a volume of
space 40 times larger than that sampled in the recent Spitzer study, which concentrated
on a region in the constellation Boötes. The Spitzer mission is designed to look at targeted
patches of sky in detail, while WISE is combing the whole sky.

"WISE is looking everywhere, so the coolest brown dwarfs are going to pop up all around
us," said Peter Eisenhardt, the WISE project scientist at NASA's Jet Propulsion
Laboratory, Pasadena, Calif., and lead author of a recent paper in the Astronomical
Journal on the Spitzer discoveries. "We might even find a cool brown dwarf that is closer
to us than Proxima Centauri, the closest known star."

Brown dwarfs form like stars out of collapsing balls of gas and dust, but they are puny in
comparison, never collecting enough mass to ignite nuclear fusion and shine with
starlight. The smallest known brown dwarfs are about 5 to 10 times the mass of our planet
Jupiter -- that's as massive as some known gas-giant planets around other stars. Brown
dwarfs start out with a bit of internal heat left over from their formation, but with age,
they cool down. The first confirmed brown dwarf was announced in 1995.

"Brown dwarfs are like planets in some ways, but they are in isolation," said astronomer
Daniel Stern, co-author of the Spitzer paper at JPL. "This makes them exciting for
astronomers -- they are the perfect laboratories to study bodies with planetary masses."

Most of the new brown dwarfs found by Spitzer are thought to belong to the coolest
known class of brown dwarfs, called T dwarfs, which are defined as being less than
about 1,500 Kelvin (2,240 degrees Fahrenheit). One of the objects appears to be so cold
that it may even be a long-sought Y dwarf -- a proposed class of even colder stars. The T
and Y classes are part of a larger system categorizing all stars; for example, the hottest,
most massive stars are O stars; our sun is a G star.

"Models indicate there may be an entirely new class of stars out there, the Y dwarfs, that
we haven't found yet," said co-author Davy Kirkpatrick, a co-author of the study and a
member of the WISE science team at the California Institute of Technology, Pasadena,
Calif. "If these elusive objects do exist, WISE will find them." Kirkpatrick is a world
expert in brown dwarfs -- he came up with L, T and Y classifications for the cooler stars.

Kirkpatrick says that it's possible that WISE could find an icy, Neptune-sized or bigger
object in the far reaches of our solar system -- thousands of times farther from the sun
than Earth. There is some speculation amongst scientists that such a cool body, if it exists,
could be a brown dwarf companion to our sun. This hypothetical object has been
nicknamed "Nemesis."

"We are now calling the hypothetical brown dwarf Tyche instead, after the benevolent
counterpart to Nemesis," said Kirkpatrick. "Although there is only limited evidence to
suggest a large body in a wide, stable orbit around the sun, WISE should be able to find
it, or rule it out altogether."

The 14 objects found by Spitzer are hundreds of light-years away -- too far away and
faint for ground-based telescopes to see and confirm with a method called spectroscopy.
But their presence implies that there are a hundred or more within only 25 light-years of
our sun. Because WISE is looking everywhere, it will find these missing orbs, which will
be close enough to confirm with spectroscopy. It's possible that WISE will even find
more brown dwarfs within 25-light years of the sun than the number of stars known to
exist in this space.

"WISE is going to transform our view of the solar neighborhood," said Eisenhardt. We'll
be studying these new neighbors in minute detail -- they may contain the nearest
planetary system to our own."

Other authors of the Spitzer paper are Roger Griffith and Amy Mainzer of JPL; Ned
Wright, A.M. Ghez and Quinn Konopacky of UCLA; Matthew Ashby and Mark
Brodwin of the Harvard-Smithsonian Center for Astrophysics, Cambridge; Mass.,
Michael Brown of Monash University, Australia; R.S. Bussmann of the University of
Arizona, Tucson; Arjun Dey of National Optical Astronomy Observatory, Tucson, Ariz.;
Eilat Glikman of Caltech; Anthony Gonzalez and David Vollbach of the University of
Florida, Gainesville; and Shelley Wright of the University of California, Berkeley.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space
Telescope mission for NASA's Science Mission Directorate, Washington. Science
operations are conducted at the Spitzer Science Center at the California Institute of
Technology in Pasadena. Caltech manages JPL for NASA.

JPL manages the Wide-field Infrared Survey Explorer for NASA's Science Mission
Directorate, Washington. The principal investigator, Edward Wright, is at UCLA. The
mission was competitively selected under NASA's Explorers Program managed by the
Goddard Space Flight Center, Greenbelt, Md. The science instrument was built by the
Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by Ball
Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing
take place at the Infrared Processing and Analysis Center at the California Institute of
Technology in Pasadena. Caltech manages JPL for NASA.

For more information about Spitzer, visit http://spitzer.caltech.edu/ and http://www.nasa.gov/spitzer . More information about WISE is online at http://wise.astro.ucla.edu and http://www.nasa.gov/wise.

#2010-210

-end-

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov

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